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A failed RPZ can dump full flow, just as if you sawed a pipe in two. You only locate them indoors where there is a good floor drain, and hopefully an alarm on the RPZ itself. If the house is highest, you use the PVB, and bring your plumbing out above grade, so all the drainage can be outdoors.

The house is the highest point on the property, which is 70' x 140'. The point at which the RPZ would be in the basement would be below the sprinkler heads in the front yard and above the sprinklers in the backyard since the house has a walk out basement.

Could I attach a drain hose to the RPZ such that the water expelled would be taken directly to a drain in the basement? Do you have any idea how much water will be expelled?

How far is the house from the street? Is the house the highest elevation on the property? how large is the property?
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High pressures and long supply lines lead to water hammer. Water hammer will cause your proposed RPZ to squirt water, so you will not be installing it indoors.

Regulator needed if 117 psi static?

I measured water pressure and flow rate in my basement. The line coming into the basement is 3/4" copper.

The measurements were made from a hose bib located just after the main shutoff. Hose bib is tee'd to the 3/4" line via a short 1/2" copper nipple.

I measured 117 psi with no water running. Then I measured 13 gpm out of the hose bib with a 7' section of 7/16" inside diameter rubber hose attached.

I have two questions...

Since the hose bib and hose is smaller than the 3/4" copper pipe, should I expect a flow rate of 13 gpm when I tee directly into the 3/4" line or will it be higher?

Do I need to install a pressure reducing valve for the sprinkler system to drop it something around 80 psi? Also read there is "straight valve with cross handle" that serves to reduce pressure. Is that something to try? It would seem the downside is that it reduces pressure but doesn't regulate.